Method for registering a component lead with a U-shaped metalized pad

ABSTRACT

A method and apparatus for printed circuit board pads with registering feature for component leads. A U-shaped metalized pad is disposed on a printed circuit board for soldering to a component lead. Solder is disposed on the pad and heated to a molten state so that surface tension and wetting effects form the molten solder into a solder mound having a U-shaped lateral cross section conforming to the U-shaped metalized pad. The solder mound has a first arm and a second arm, and a lateral aperture extending therebetween for receiving an extremity of the component lead, and registering the extremity of the component lead with respect to the pad.

FIELD OF THE INVENTION

The present invention generally relates to the field of electronicassembly and more particularly relates to electronic componentinterconnects in electronic assemblies.

BACKGROUND OF THE INVENTION

Electronic assemblies typically employ printed circuit boards formechanical support and electrical interconnection of electroniccomponents. In the case of surface mount electronic components,component leads are placed onto metal pads deposited onto a surface ofthe boards. Solder is used to attach the components leads to the padsand to electrically connect the components leads to the pads. The padsare electrically interconnected by traces, or connected to an internalconductive plane of the circuit board through a metal plated apertureextending into the circuit board, called a via, so as to provide forelectrical interconnection of the components.

In an assembly process, solder paste is placed onto the pads on a firstside of the printed circuit board. Components are arranged on the firstside of the circuit board so that component leads cling to the solderpaste in appropriate locations. The paste is heated to solder thecomponents to the pads on the first side of the printed circuit board.The solder is allowed to cool and harden.

After the solder on the first side of the printed circuit board ishardened, enhanced component density is achieved by turning the printedcircuit board over, placing solder paste onto the pads on the other sideof the printed circuit board and arranging components on the other sideof the circuit board so that component leads cling to the solder pastein appropriate locations. The paste is heated to solder the componentsto the pads on the other side of the printed circuit board. The solderis again allowed to cool and harden.

It should be understood that flexibility in order of solderingoperations is limited since the solder paste on the first side of thecircuit board is heated, cooled and hardened before any solder paste isapplied to the other side of the printed circuit board. If solder pasteand components were applied to both sides of the printed circuit boardbefore any heat were applied to either side, it is likely that the pullof gravity or vibrations would displace some of the components, sincethe solder paste has insufficient adhesive power to retain positioningof components so effected. Flexibility in order of soldering operationsis particularly limited when the components include components that mustbe soldered to pads on both sides of the printed circuit board, such asstraddle mount connectors.

It is important to achieve and maintain correct positioning ofcomponents in soldering operations. Misregistered component leads causeelectrically or mechanically defective solder joints, requiring costlyand time consuming rework operations. Misregistered component leads mayalso cause circuit shorting and catastrophic electrical failure of theentire electronic assembly.

What is needed is a method and apparatus for printed circuit board padswith registering feature for component leads.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for printedcircuit board pads with registering feature for component leads.

Briefly, and in general terms the invention provides a U-shapedmetalized pad on a printed circuit board for soldering to a componentlead. Solder is disposed on the pad and heated to a molten state so thatsurface tension and wetting effects form the molten solder into a soldermound having a U-shaped lateral cross section conforming to the U-shapedmetalized pad. The solder mound has a first arm and a second arm, and alateral aperture extending therebetween for receiving an extremity ofthe component lead. The solder mound also has a bridge portion extendingbetween the first and second arms of the solder mound. The solder moundis cooled so as to harden the solder mound.

The extremity of the component lead is disposed proximate to the lateralaperture between the first arm and second arm of the solder mound. Bysliding the extremity of the component lead in contact with the firstarm and second arm of the solder mound, the component lead is registeredwith respect to the pad in a lateral dimension perpendicular to the armsof the mound. The lateral aperture of the solder mound receives theextremity of the component lead. By sliding the extremity of thecomponent lead into contact with the bridge portion of the solder mound,the component lead is registered with respect to the pad in a lateraldimension perpendicular to the bridge portion of the solder mound. Thesolder mound is then heated to a molten state so as to solder thecomponent lead to the pad.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 2A-C, and 3A-C show various views of solder pad researchinvestigations.

FIGS. 4A and 4B show a detailed plan and front view of a U-shapedmetalized pad of a preferred embodiment of the invention.

FIGS. 4C and 4D show a detailed plan and front view of solder pastedisposed on the pad of FIGS. 4A and 4B.

FIGS. 4E and 4F show a detailed plan and cross sectional view of apreferred solder mound of the invention conforming to the pad of FIGS.4A and 4B.

FIGS. 5A-5C to 11A-11C are various views illustrating a registrationfeature of the invention.

FIG. 12A is a cross sectional view of the preferred embodiment of theinvention.

FIGS. 12B and 12C are cross sectional views of alternative embodimentsof the invention.

FIGS. 13A-13D are plan views illustrating various preferred dimensionsof the pad and solder mound of the invention.

FIG. 14 is a plan view of a preferred array of the invention.

FIG. 15 is a side view of the preferred embodiment of the invention.

FIG. 16 is an isometric view of the preferred embodiment of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

FIGS. 1A, 1B, 2A-C, and 3A-C show various views of solder pad researchinvestigations. In an assembly process, solder paste is placed onto thepads and is melted or “reflowed” into a molten state. A wetting effectspreads the molten solder out to pad edges so as to cover a respectivesurface of each pad. A surface tension effect causes the molten solderto form a respective solder mound 2 covering each pad, 4 for example asshown in plan view and cross sectional view in FIGS. 1A and 1B. Thesolder mound is allowed to cool and harden. After component placement ontop of the solder mounds, heat is applied to reflow the solder, so as tosolder the component leads to the pads.

Leads of each surface mount component are placed onto the topsrespective solder mounds of the pods using a combination of motions. Insome cases, components are set down onto the board and then pushedlaterally using sliding motions across the surface of the board, untilleads of each component are in correct lateral position with respect tothe pads. Alternatively, components are laterally positioned whilesubstantially above the surface of the board, and then placed onto padsusing a motion that is perpendicular to a surface of the board. Socalled “pick and place machines” are easily adjusted for correctperpendicular motion, but adjustment for correct lateral positioning islimited.

For correct lateral positioning, the component leads are registered withthe pads by precisely adjusting position of each component lead incorrespondence with the respective pad. Correct lateral positioning issometimes difficult to achieve and maintain since weight of thecomponents, undesired vibrations, or resiliency of the component leadscause the component leads to slip off the tops of the solder mounds,slide down sloped sides of the solder mounds, and misregister withrespect to the pads.

For example, as shown in plan view, cross sectional view, and side viewin FIGS. 2A, 2B, and 2C, a component lead 6 is slid towards the soldermound covering the pad. However correct lateral positioning is notachieved or maintained because the component lead slips off the top ofthe solder mound, slides down sloped side of the solder mounds, andmisregisters with respect to the pad, as shown in plan view, crosssectional view, and side view in FIGS. 3A, 3B, and 3C.

It is important to achieve and maintain correct lateral positioning ofcomponents in soldering operations. Misregistered component leads causeelectrically or mechanically defective solder joints when heat isapplied to reflow the solder, requiring costly and time consuming reworkoperations. Misregistered component leads may also cause circuitshorting and catastrophic electrical failure of the entire electronicassembly.

FIGS. 4A and 4B show a detailed plan and front view of a U-shapedmetalized pad 101 of a preferred embodiment of the invention. The pad101 is preferably plated, deposited, or otherwise disposed on a printedcircuit board for soldering to a component lead. Solder paste 103 isdisposed on the pad 101 as shown in detailed plan and front views inFIGS. 4C and 4D. The solder paste 103 is heated to a molten state sothat surface tension and wetting effects form the molten solder into asolder mound 105 having a U-shaped lateral cross section and conformingto dimensions of the U-shaped metalized pad 101 as shown in plan andcross-sectional views in FIGS. 4E and 4F. Accordingly, just as theU-shaped pad 101 has a first arm 121, and a second arm 123, and alateral aperture 125 extending therebetween for receiving an extremityof the component lead, so also does the solder mound 105 have a firstarm 131 and a second arm 133, and a lateral aperture 135 extendingtherebetween for receiving the extremity of the component lead andopposing the lateral aperture. Just as the U-shaped pad 101 has a bridgeportion 127 extending between the first and second arms 121, 123 of thepad 101, so also does the solder mound 105 have a bridge portion 137extending between the first and second arms 131, 133 the solder mound.The solder mound 105 is cooled so as to harden the solder mound 105.

Preferably, sufficient solder paste is used so that the solder mound hasa height dimension within a range of five thousandths (0.005) of an inchto twenty thousands (0.020) of an inch. It is theorized that if theheight dimension of the solder mound is substantially less than fivethousandths (0.005) of an inch, there will be an insufficient amount ofsolder and a corresponding solder joint will lack mechanical strength.It is theorized that if the height dimension of the solder mound issubstantially greater than twenty thousandths (0.020) of an inch, thenexcess solder may cause undesirable solder bridging to adjacent pads.Preferably, the height dimension the solder mound is approximately tenthousandths (0.010) of an inch.

As shown in plan view, cross sectional view, and side view in FIGS. 5A,5B, and 5C, the extremity 141 of the component lead 143 is disposedproximate to the lateral aperture between the first arm and second armof the solder mound. By sliding the extremity 141 of the component lead143 in contact with the first arm and second arm of the solder mound,the component lead is registered with respect to the pad in a lateraldimension perpendicular to the arms of the mound, as shown in plan view,cross sectional view, and side view in FIGS. 6A, 6B, and 6C. The lateralaperture of the solder mound receives the extremity of the componentlead. By sliding the extremity of the component lead into contact withthe bridge portion of the solder mound, the component lead is registeredwith respect to the pad in a lateral dimension perpendicular to thebridge portion of the solder mound. Preferably, dimensions of the padand solder mound are selected so that a main angled contact point of theextremity of the component lead is centrally disposed with respect tothe pad and solder mound upon registration with the arms and bridgeportion, as shown in FIGS. 6A, 6B, and 6C.

The solder mound is then heated to a molten state so as to solder thecomponent lead to the pad. The registering feature of the invention isparticularly advantageous, since it is important to achieve and maintaincorrect lateral positioning of the component lead with respect to thepad. Misregistered component leads would cause electrically ormechanically defective solder joints when heat is applied to reflow thesolder, requiring costly and time consuming rework operations.

Aspects of the registering feature of the preferred embodiment of theinvention are further shown in FIGS. 7-11. In accordance with theprinciples of the invention for correct lateral positioning, aregistering feature of the invention provides for the component leadregistering with the pad by precisely adjusting position of eachcomponent lead in correspondence with the pad. An example of a componentlead that is laterally mis-aligned with respect to the pad and soldermound as shown in plan view, cross sectional view, and side view inFIGS. 7A, 7B, and 7C. As shown in plan view, cross sectional view, andside view in FIGS. 8A, 8B, and 8C, 9A, 9B, and 9C and 10A, 10B, and 10C,the extremity of the component lead slides in contact with the soldermound, engaging the first arm of the solder mound for correcting lateralalignment of the component lead with respect to the pad and soldermound. Resiliency of the component lead causes the component lead toslide along a sloped side of the arm of the solder mound, and laterallyregister with respect to the pad in the lateral aperture of the soldermound. As shown in plan view, cross sectional view, and side view inFIGS. 11A, 11B, and 11C in the preferred embodiment the sliding motionof the extremity of the component lead continues so as to engage boththe first and second arm of the solder mound, so that the component leadis registered with respect to the pad in a lateral dimensionperpendicular to the arms of the mound, as shown.

For preferred solder wetting of the component lead by both arms of thesolder mound, width dimension of the lateral aperture of the soldermound (and the pad) is substantially the same as a width dimension ofthe component lead, so that the lateral aperture of the solder moundreceives the extremity of the component lead as shown in cross sectionalview in FIG. 12A. However it should be understood that the invention isnot strictly limited to the lateral aperture of the solder mound beingsubstantially the same as the width dimension of the component lead. Forexample, in an alternative embodiment as shown in cross sectional viewin FIG. 12B, the width dimension of the lateral aperture of the soldermound is substantially larger than the width dimension of the componentlead. In another alternative embodiment as shown in cross sectional viewin FIG. 12C, the width dimension of the lateral aperture of the soldermound is substantially smaller than the width dimension of the componentlead.

Preferably the width dimension of the lateral aperture of the soldermound (and the pad) is within a range of approximately five thousandths(0.005) of an inch to approximately twenty thousandths (0.020) of aninch. It is theorized that if the width dimension of the lateralaperture of the solder mound (and the pad) is substantially less thanfive thousandths (0.005) of an inch, then the corresponding width of thecomponent lead received by the lateral aperture would be too thin andtherefore lacking in mechanical strength. It is theorized that if thewidth dimension of the lateral aperture of the solder mound (and thepad) is substantially greater than twenty thousandths (0.020) of an inchthen the corresponding width of the component lead received by thelateral aperture would be too thick and therefore limit componentdensity. Preferably, the width dimension of the lateral aperture of thesolder mound (and the pad) is approximately ten thousandths (0.010) ofan inch.

As shown in plan view in FIG. 13A, preferably the lateral aperture ofthe solder mound (and the pad) has a tapered portion, represented as aninitial flared portion that is wider than the width of the lateralaperture, so as to promote capture and registration of the extremity ofthe component lead. Preferably, a width dimension of the initial flaredportion of the lateral aperture of the solder mound (and the pad) iswithin a range of approximately twenty thousandths (0.020) of an inch toapproximately forty thousandths (0.040) of an inch. It is theorized thatif the width dimension the initial flared portion of the lateralaperture of the solder mound (and the pad) is substantially less thantwenty thousandths (0.020) of an inch, then capture and registration ofthe component lead may be limited in some instances. It is theorizedthat if the width dimension of the initial flared portion of the lateralaperture of the solder mound (and the pad) is substantially greater thanforty thousandths (0.040) of an inch, then pad and component densitywould be limited. Preferably, the width dimension of the initial flaredportion of the lateral aperture of the solder mound (and the pad) isapproximately thirty thousandths (0.030) of an inch.

To promote capture and registration of the extremity of the componentlead, a flare angle of the initial flared portion of the lateralaperture of the solder mound (and the pad) is within a range ofsubstantially more than zero degrees to substantially less than ninetydegrees. Preferably, the flare angle of the initial flared portion ofthe lateral aperture of the solder mound (and the pad) is approximatelyforty-five degrees, as shown in plan view in FIG. 13B.

Preferably the width dimension of each arm of the solder mound (and thepad) is within a range of approximately three thousandths (0.003) of aninch to approximately fifteen thousandths (0.015) of an inch, as shownin plan view in FIG. 13C. It is theorized that if the width dimension ofeach arm of the solder mound (and the pad) is substantially less thanthree thousandths (0.003) of an inch, then there will be insufficientsolder wetting of the pad and component lead. It is theorized that ifthe width dimension of each arm of the solder mound (and the pad) issubstantially greater than fifteen thousandths (0.015) of an inch, thenpad and component density would be limited. Preferably, the widthdimension of each arm of the solder mound (and the pad) is approximatelyten thousandths (0.010) of an inch.

Preferably the length dimension of each arm of the solder mound (and thepad) is within a range of approximately twenty thousandths (0.020) of aninch to approximately fifty thousandths (0.050) of an inch, as shown inplan view in FIG. 13D. It is theorized that if the length dimension ofeach arm of the solder mound (and the pad) is substantially less thantwenty thousandths (0.020) of an inch, then capture and registration ofthe extremity of the component lead may be limited in some instances. Itis theorized that if the length dimension of each arm of the soldermound (and the pad) is substantially greater than fifty thousandths(0.050) of an inch, then pad and component density would be limited.Preferably, the length dimension of each arm of the solder mound (andthe pad) is approximately thirty-five thousandths (0.035) of an inch.

Preferably the width dimension of the bridge portion of the solder mound(and the pad) is within a range of approximately five thousandths(0.005) of an inch to approximately twenty thousandths (0.020) of aninch, as shown in plan view in FIG. 13D. It is theorized that if thewidth dimension of the bridge portion of the solder mound (and the pad)is substantially less than five thousandths (0.005) of an inch, thenthere will be insufficient solder wetting of the pad and component lead,and insufficient registration as the extremity of the component leadcontacts the bridge portion. It is theorized that if the width dimensionof the bridge portion of the solder mound (and the pad) is substantiallygreater than twenty thousandths (0.020) of an inch, then pad andcomponent density would be limited. Preferably, the width dimension ofthe bridge portion of the solder mound (and the pad) is approximatelyten thousandths (0.010) of an inch.

As shown in plan view in FIG. 14, in the preferred embodiment of theinvention an array of the U-shaped metalized pads is deposited on asurface of the printed circuit board 100. In the preferred embodimentU-shaped metalized pads are arranged in one or more rows, and eachU-shaped metalized pad of the array is similarly oriented. Preferably,the pads are arranged in a spaced apart relation with a range ofapproximately thirty thousandths (0.030) of an inch to approximately onehundred thousandths (0.100) of an inch. It is theorized that if thespaced apart relation is substantially less than thirty thousandths(0.030) of an inch, undesirable solder bridging between pads may result.It is theorized that if the spaced apart relation is substantiallygreater than one hundred thousandths (0.100) of an inch, then pad andcomponent density would be limited. Preferably, the spaced apartrelation is approximately fifty thousandths (0.050) of an inch.

Solder paste is applied to the pads of a first side of the printedcircuit board in a manner as discussed in detail previously herein. Thesolder paste is heated to a molten state so that surface tension andwetting effects form the molten solder into a plurality of solder moundsthat each contact a respective one of the U-shaped metalized pads, thesolder mounds each having a respective U-shaped lateral cross sectionconforming to the respective U-shaped metalized pad. Each solder moundincludes a respective first arm and second arm, a respective lateralaperture extending between the arms of each solder mound for receivingan extremity of a respective component lead; and a respective bridgeportion extending between the arms of each solder mound.

A particularly advantageous use of the invention is in conjunction witha Surface mount component, for example a straddle mount connector 150 asshown in side view in FIG. 15 and isometric view in FIG. 16, having acomponent lead for soldering to a pad on a first surface of the printedcircuit board, and further having a component lead for soldering to apad on an opposing surface of the printed circuit board. As shown, theU-shaped pads are preferably deposited on the surface of the printedcircuit board proximate to an edge of the printed circuit board, so asto provide for advantageous use in conjunction with the connector.

The present invention provides a method and apparatus for printedcircuit board pads with registering feature for component leads.Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrate, and variousmodifications and changes can be made without departing from the scopeand spirit of the invention. Within the scope of the appended claims,therefore, the invention may be practiced otherwise than as specificallydescribed and illustrated.

What is claimed is:
 1. A method comprising: providing a solder moundhaving a U-shaped lateral cross section disposed along a surface of aprinted circuit board, the solder mound having a lateral bridge portion,and having both first and second arm extremities extending in alongitudinal direction away from the bridge portion along the surface ofthe printed circuit board; tapering the first and second arm extremitiesof the solder mound in the longitudinal direction away from the bridgeportion along the surface of a printed circuit board, so as to formtapered solder mound extremities and a tapered solder mound aperturelaterally therebetween; disposing a component lead proximate to thetapered solder mound aperture; and registering the component lead in thetapered solder mound aperture along the tapered solder moundextremities.
 2. A method as in claim 1 wherein the step of registeringthe component lead includes sliding the component lead along the surfaceof the printed circuit board in contact with at least one of the taperedsolder mound extremities, so that the tapered solder mound aperture ofthe solder mound receives the component lead.
 3. A method as in claim 1wherein the step of registering the component lead includes sliding thecomponent lead along the surface of the printed circuit board in contactwith both of the tapered solder mound extremities, so that the taperedsolder mound aperture of the solder mound receives the component lead.4. A method as in claim 1 wherein the step of registering the componentlead includes sliding the component lead into contact with the lateralbridge portion of the solder mound, so as to register the component leadwith respect to the lateral bridge portion of the solder mound.
 5. Amethod comprising: providing a solder mound having a U-shaped lateralcross section disposed along a surface of a printed circuit board, thesolder mound having both first and second arm extremities extending in alongitudinal direction along the surface of the printed circuit board;tapering the first and second arm extremities of the solder mound in thelongitudinal direction away from the bridge portion along the surface ofa printed circuit board, so as to form tapered solder mound extremitiesand a tapered solder mound aperture laterally therebetween; disposing acomponent lead proximate to the tapered solder mound aperture; laterallyregistering the component lead in the tapered solder mound aperturealong the tapered solder mound extremities.